1. Field of the Invention
The present invention relates to a zoom lens applicable as an optical system to a digital camera, a camera device and a data terminal device incorporating such a zoom lens.
2. Description of the Related Art
Along with widespread of a digital camera generally including a zoom lens and an autofocus (AF) function, there have been users' strong demands for development of a high performance, compact-size, high-speed autofocus camera.
To downsize the zoom lens, it is necessary to shorten the total length of the zoom lens in use from the lens surface closest to an object to an image plane. To improve AF speed, it is essential to downsize a focus lens.
To improve the performance of the zoom lens for a high-end digital camera, it is necessary to exert resolution in the entire zoom range to be able to deal with an image sensor with at least 5 to 10 million pixels. A known zoom lens with high magnification includes first to fifth lens groups having positive, negative, negative, positive, and positive focal lengths in this order from an object side, respectively.
Japanese Patent No. 4401451 (Reference 1) and No. 3716418 (Reference 2), for example, disclose such a zoom lens made of the five lens groups. In the zoom lens in Reference 1, the third lens group is a focus lens made of a cemented lens of positive and negative lenses. This may cause a problem that it is difficult to reduce the third lens group in weight and size and requires a large load and a large driving motor to move the focus lens. Further, it is difficult to shorten time taken for focusing and heighten the AF speed without great driving power.
Likewise, the third lens group in the zoom lens disclosed in Reference 2 is also made of three negative, positive, negative lenses so that it faces a problem in terms of decreasing its size and weight and increasing the AF speed.